In a cognitive radio system, a cognitive secondary radio will utilize spectrum assigned to a primary system using an opportunistic approach. With this approach, the secondary radio system will share the spectrum with primary incumbents as well as those operating under authorization on a secondary basis. Under these conditions, it is imperative that any user in the cognitive radio system not interfere with primary users. Some types of cognitive radio systems (e.g., IEEE 802.22) require that devices sense the channel to detect a licensed, primary user. The devices are allowed to transmit if their transmissions will not interfere with any primary user. This is generally accomplished by the secondary user determining a signal strength of the primary users on a particular channel, and if the signal of any primary user is above a predetermined threshold, the cognitive radio device determines that its transmissions on that channel would cause interference to the primary user, and so inhibits transmission.
Many cognitive radio proposals are based on a multi-carrier (e.g. Orthogonal Frequency Division Multiplexed OFDM)) physical layer due to their inherent frequency shaping ability. Data can be transmitted on most of the carriers while others are left blank to reduce interference to specific frequency bands. To maximize throughput and avoid data loss, the receiver should have a priori knowledge of the carrier formatting.
A problem exists in that sometimes the receiver may not know the carrier formatting (i.e., what carriers are being utilized for transmission) prior to frame reception. For example, if a calling channel is used to initiate communication between two devices, the transmitting device will first perform spectrum measurements to determine which carriers need to be empty and which can carry data. In the absence of other handshaking messages, the transmitting device has no way to let the receiver know the resulting carrier format prior to the calling channel request. What is needed is a method and apparatus for communication by a secondary user of spectrum by which the receiver can demodulate transmissions without a priori knowledge of carrier format.